What does "conversion never fails at run time" mean?

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I am into Chapter 2 of The Go Programming Language and under a section Type Declarations there is an excerpt that says:

Conversions are allowed between numeric types, and between string and some slice types, as we will see in the next chapter. These conversions may change the representation. For instance, converting a floating-point number to an integer discards any fractional part, and converting a string to a []byte slice allocates a copy of the string data. In any case, a conversion never fails at run time.